When transmitting or storing digital data in the form of digital data bits, bit errors are normally scarcely avoidable. Frequently, the bit errors are random. An opportunity for recognizing and possibly even correcting bit errors in transmitted or stored data bits is provided particularly by redundancy-based error recognition methods and error correction methods. These are based on further data being transmitted and stored in addition to the actual data bits, which further data reveal whether a bit error is present and can possibly be corrected within the data bits.
Bit patterns which represent data can also be obtained using a physical hash function (PUF). The idea on which a physical hash function is based is to digitize physical properties of an object and thus to obtain a bit succession (number or digit) which is associated with the object.
In this case, it is desirable for the bit successions of two different physical objects to be uncorrelated to one another. A simple example for the purposes of illustration is a sheet of paper. When viewed under a microscope, it is possible to see a specific fine structure of the wood chips or pulp portions. The structure is measured using a suitable algorithm, and the result is represented as a bit succession. This bit succession is the PUF. Another sheet of paper generally produces a bit succession which is uncorrelated to the bit succession of the first sheet.
Using the physical hash function or physically unclonable function (PUF), it is now possible to determine, at a first time, a bit pattern which is characteristic of the physical properties of the respective object to the first time. By way of example, this bit pattern can be used to encrypt useful data. In order to decrypt the useful data again at a later time, the same bit pattern is required again. The bit pattern required can in turn also be obtained at the second time from the physical properties of the object, but with slight discrepancies from the bit pattern produced at the first time being expected. As the interval of time between the first and second times increases, the physical properties of the object can alter permanently, which results in particular discrepancies in the bit pattern occurring ever more frequently and finally almost always. Added to the age-related discrepancies are typically also random discrepancies between the bit patterns generated at the first time and at the second time, which means that after a particular time it is no longer possible to ensure reliable correction of the bit pattern.